The original single site catalysts of the mid 1980's, such as a metallocene catalyst, produced resin having a narrow polydispersity (Mw/Mn) typically in the range from about 2.5 to 3.5. Early on it was recognized that either blending such resins or the use of different metallocene catalyst, in the same reactor could produce bimodal resins, each component having a narrow polydispersity and the blend having a broader polydispersity. It was felt such resins would provide a good balance of processability and physical properties such as resin toughness. There are an increasing number of patents and applications in this field.
U.S. Pat. No. 4,530,914 issued Jul. 23, 1985 to Ewen et al., assigned to EXXON Research & Engineering Co. teaches the use in the same reactor of two metallocene catalysts each having different propagation and termination rate constants for ethylene polymerizations. The catalyst combination taught in the patent is not the same as that contemplated by the present invention.
There are a number of patents wherein a bimodal resin is produced having a controlled molecular weight distribution by using different single site catalyst such as metallocene in two or more tandem reactors. United States patent application 2002/0045711 in the name of Backman et al., published Apr. 18, 2002 is illustrative of this type of art. The reference teaches away from the present invention in that the present invention contemplates the use of a single reactor, not tandem reactors.
U.S. Pat. No. 6,309,997 issued Oct. 30, 2001 teaches an olefin polymerization catalyst using a phenoxide (preferably a salicylaldimine) ligand for use in the polymerization of olefins. The patent does not teach the use of mixed catalysts systems nor does it teach process control to adjust the polymer characteristics such as bimodality and comonomer incorporation.
United States patent application 2002/0077431 published Jun. 20, 2002 in the name of Whiteker discloses a process for the polymerization and oligomerization of olefins in the presence of a mixed catalyst system in a single reactor. The catalyst system as disclosed comprises a first component similar to the first component in the catalyst system of the present invention except that at least one of substituents R3, R4, R5, R8, R9 and R10 must have a Hammett σρ value (Hansch et al., Chem Rev. 1991,91,165) greater than 0.2 (i.e. at least one of these substituents needs to be a sufficiently electron withdrawing group, (e.g. CF3, Br, etc.)). In the process according to the present invention all R3, R4, R5, R8, R9 and R10 are hydrocarbyl substituents which have a Hammett value of less than 0.2. Further, the reference fails to teach or suggest the molecular weight distribution of the components in the resulting polymer may be altered or controlled by altering or controlling the reaction conditions.
The present invention seeks to provide a relatively simple method for controlling the ratio of the molecular weight distribution of a bimodal resin and optionally the comonomer placement or distribution in a bimodal resin produced in a single gas phase reactor in the presence of a mixed catalyst system on the same support by controlling one or more steps selected from the group consisting of:
(a) altering the temperature of the reaction by at least 1° C. within the range from 50 to 120° C.;
(b) altering the partial pressure of the hydrogen component of the reaction mixture by at least 0.02 psi (0.138 KPa);
(c) altering the partial pressure of ethylene in the reaction mixture by not less than 10 psi (68.94 KPa); and
(d) altering the amount of non polymerizable hydrocarbon in the reaction mixture by not less than 0.5 mole %.